1. Field of the Invention
This invention relates to a method for incoporating "tag" materials within nuclear reactor components in order to readily identify the specific components in the event they fail. In particular, this invention relates to a method for using solid materials which generate tag gases when exposed to radiation in a nuclear reactor.
2. Description of the Prior Art
Nuclear reactor components, particularly fuel elements, are often constructed with gaseous isotopes sealed within the component assembly that are released in the event of a component failure. The gaseous isotopes are selected such that they can be easily detected and identified; thus signaling the failure and identifying, specifically, the failed component.
Reactor fuel elements, also known as fuel rods and fuel pins, are introduced into a nuclear reactor in groups as fuel assemblies, or bundles, with each fuel assembly containing as many as 100 to 400 fuel elements, depending on the size of the reactor. A single reactor may have 100 or more fuel assemblies. Each fuel assembly may be tagged with a particular isotope or a mixture of isotopes in a known ratio. The detection of an isotope or a specific mixture of isotopes signals the failure of one or more fuel elements and identifies the assembly in which the failure occurred.
Four elements may be of two types of construction. They may be vented, wherein gaseous fission products are vented into the liquid reactor coolant and collected in the reactor cover gas, or they may be closed, wherein the gaseous fission products are collected in a gas plenum provided within the sealed fuel element. Tags can only be used with sealed elements.
U.S. Pat. No. 3,663,363, issued to Crouthamel et al., discloses gaseous tags consisting of various mixtures of xenon isotopes having mass numbers less than 131. These tags are added in steel capsules to closed fuel elements with fuel pellets, helium is then added as a heat transfer medium and the elements are sealed. The capsules are then punctured with a device included with each capsule, releasing the tag gas into the sealed element. In the event that an element fails, the gaseous xenon isotope mixture is detected in the reactor cover gas and the location of the failure is determined from the specific ratio of isotopes. Xenon isotopes with mass numbers from 124 and 130 were selected for use as tags as they provided the preferred tag characteristics of not being produced to any significant degree during fission, not having an adverse effect on neutron flux, being unaffected by radiation, being transported to the cover gas with fission product gases, and being easily identified using a mass spectrometer. The preparation of the isotope gas capsules is an exacting and expensive procedure; wherein the gases are mixed in a predetermined ratio, the capsules are placed in a closed chamber, the chamber is evacuated, the tag gas mixture is introduced into the evacuated chamber and, concomitantly, into the capsules, and the capsules are sealed by lazer welding.
In addition to using xenon 124 and 130 isotopes, it is also practical to use isotopes of krypton as tag gases. However, the use of isotopic gas tags is a costly procedure as more and more unique ratios must be used, introducing problems of isotopic enrichment. There are also problems in maintaining tag gas purity during the placement of the isotope mixture into fuel elements and other reactor components.
It is an object of the present invention to provide a method and materials for tagging fuel elements and other reactor components which avoids the mechanical difficulties and high costs encountered in assembling devices which contain gaseous isotopes while maintaining the purity of the tag mixtures.
Additional objectives of the invention are to provide tag materials that are easily handled, that can be used separately or in conjunction with gas tags, that do no result in fission products, and that result in stable end products present in ratios which are different from their relative occurrance in nature.